The invention discloses a method for preparing a polymer by micro-scale photo-induced organic catalysis. The method comprises the following steps: using alkyl halide as an initiator in a microreactor; and using an organic small molecule as a catalyst to catalyze atom-transfer radical-polymerization, so as to prepare the polymer. Compared with the prior art, the method provided by the invention has advantages of high reaction speed, controllable molecular weight and molecular weight distribution of a polymerization product, no residue, and wide application in fields of optoelectronic materials and biomedical materials.